1. Field of the Invention
The present invention relates to an image-forming apparatus, an image-forming method, a developing apparatus, and a developing method each utilizing electrophotography.
2. Description of the Related Art
Although many methods have each been known as electrophotography, a general method is as described below. An electrostatic latent image is formed on an electrostatic image carrier with a photoconductive material by various units. Next, the electrostatic latent image is developed with toner to be turned into a visible image and the toner image is transferred onto a recording medium such as paper as required. After that, the toner image is fixed onto the recording medium by heat, a pressure, or the like to provide a copy. A copying machine, a printer, or the like is available as such image-forming apparatus.
The transition of such printer or copying machine from an analog system to a digital system has progressed in recent years, and hence the printer or the copying machine has been strongly required to have excellent latent image reproducibility and a high resolution. At the same time, the downsizing of, in particular, the printer has been strongly required.
The printer has heretofore been used in the following manner in many cases. The printer is connected to the network and many persons perform printing with the printer. However, a demand for the following manner has been growing in recent years. A PC and the printer are placed on an individual's desk, and the individual performs printing at hand. To this end, the space savings of the printer is necessary and hence the requirement for the downsizing of the printer has been strong.
In addition, even such compact printer has been strongly desired to have high image quality and such high durability that a variation in image quality is small even when the printer is used for a long time period.
Here, attention is paid to the downsizing of the printer. The downsizing of a fixing device and the downsizing of a developing apparatus are mainly effective for the downsizing. In particular, the latter accounts for a considerable portion of the volume of the printer and hence the downsizing of the developing apparatus can be said to be essential for the downsizing of the printer.
A development system is considered. A two-component development system or a one-component development system is available as the development system of the printer, but one-component development is suitable in a sense that a compact printer can be achieved. This is because no carrier is used.
Next, downsizing in the one-component development is considered. A reduction in diameter of an electrostatic latent image bearing member or a toner carrier is effective for the downsizing of the developing apparatus.
In addition, a cleaner-less system is extremely suitable for the downsizing of the developing apparatus. In many printers, toner on the electrostatic latent image bearing member remaining after a transferring step (hereinafter referred to as “transfer residual toner” or “residual toner”) is scraped off by a cleaning blade or the like and recovered in a cleaner container. However, the cleaner-less system can significantly reduce the size of the developing apparatus because the system is free of the cleaning blade and the cleaner container.
Meanwhile, the cleaner-less system involves its own peculiar problems. One of the problems is the recoverability of the residual toner. In the cleaner-less system, the residual toner is recovered in the toner carrier in an area where the toner carrier and the electrostatic latent image bearing member abut with each other (hereinafter referred to as “development area”). However, when the recoverability of the residual toner is poor, the toner remains while being carried on the electrostatic latent image bearing member, which causes an image defect such as an increase in fog or a cleaner-less ghost. Here, the cleaner-less ghost is a phenomenon in which the residual toner after transfer is not recovered in a developing portion and is transferred in a following transferring step.
Attention is paid to the recoverability. In a low-temperature and low-humidity environment, the charge-up of the toner is liable to occur and the adhesive force of the toner to the electrostatic latent image bearing member increases. As a result, the recoverability of the residual toner tends to deteriorate.
To solve such problems, it has been reported that, for example, when a direct-current voltage is superimposed on an alternating voltage of a developing bias or the circularity and residual magnetization of the toner are controlled, even the cleaner-less system can maintain good image quality in a normal-temperature and normal-humidity environment (see Japanese Patent Application Laid-Open No. 2005-173484 and Japanese Patent Application Laid-Open No. 2006-154093).
However, even when such developing apparatus and toner are used, the recoverability in the low-temperature and low-humidity environment has been susceptible to improvement.
On the other hand, when the charge quantity of the toner is reduced for reducing the adhesive force between the toner and the electrostatic latent image bearing member with a view to improving the recoverability in the low-temperature and low-humidity environment, fog worsens particularly in a high-temperature and high-humidity environment. In general, fogging toner is hardly transferred because its charging polarity is opposite to regular charging polarity or its charge quantity is substantially zero. In addition, in the cleaner-less system, a mechanism for scraping off the residual toner such as a cleaning blade is absent and hence the residual toner contaminates a charging member. When the charging member is contaminated, the same potential as a preset one (potential on the electrostatic latent image bearing member) cannot be obtained and hence the fog tends to be additionally liable to increase.
As described above, in the cleaner-less system, it has been difficult to achieve compatibility between the recoverability of the residual toner in the low-temperature and low-humidity environment, and the suppression of the fog in the high-temperature and high-humidity environment, and hence the system has been susceptible to further improvement.